How to "erase" some of my sculpting
That is impossible. Blender does not store a record of the previous location of individual vertices. Once you have sculpted an area, it edits the original mesh.

There are many workarounds though.

The most effective (and in a way, easy) method would be to select the area you would like to undo (in edit mode) and delete it. You can then remake it, or go back to a previous save/version you have created, and copy back that section. For this method to be effective you need to save a version of your project before you begin sculpting.

However, if you are going to begin a new model from scratch, I would suggest one of these methods.

First, you could create a gray-scale texture, and paint on this, using it as a displacement for the original mesh. You then "sculpt" in texture paint mode.
The main downfall to this is that it cannot auto-update. You need to switch to edit mode and back to see the changes take effect.

The best method

You can model your object, then duplicate it (make sure it is unwrapped). Now sculpt on the duplicate. when you are done (or want to undo something), you can bake the normal map for the sculpt onto the original. (there are lots of tutorials on baking normal maps.) Now use this map in a displacement modifier (on the original mesh) with "direction" set to "RGB to XYZ".



With either of these methods, to remove a portion, use a vertex group to "mask out" the results.

(Using these methods you will need a subsurf modifier to enable the displacement, or to subdivide your original model for more control.)

Here are some more "corrective" options



Use the "smooth" tool.



You can also hold ctrl which generally does the "opposite" of whichever tool you are using.

Otherwise you can use undo.